Notwithstanding that the smart-match operator, the crowning jewel of Perl6, is going to do about 15 times as many things as it currently does in Perl5, Perl6's trends are not particularly relevant to Perl5 development. It is not the goal of Perl5 to have it gradually become Perl6.

If such a join existed, every time I saw it I would have to stop and think to myself, "oh, this is really a reduce with an odd name"

Do you really believe that you have no ability to assimilate idioms into your understanding? Do you still see a map and think "Oh, that's really a foreach with an odd name and an accumulator"?

I have updated the original post to address the issue of reduce as a built-in, and some other issues that have been raised (including a way of understanding the proposed form of join).